FIG. 5 illustrates an example of conventional construction of a wheel-drive bearing unit in which a rolling bearing unit for wheel support, which is one kind of a rolling bearing unit that is the object of the present invention, is assembled. A wheel-drive bearing unit includes a wheel-support bearing unit 1 and a universal joint outer ring 2. The wheel-support bearing unit 1 includes an outer ring 3, a hub 4, and plural rolling bodies 5.
A stationary-side flange 6 is provided around the outer-circumferential surface of the outer ring 3, and double-row outer-ring raceways 7a, 7b are provided around the inner-circumferential surface of the outer ring 3. The hub 4 includes a main hub body 8 and an inner ring 9. A rotating-side flange 10 is provided around a portion near the end section on one side in the axial direction of the outer-circumferential surface of the main hub body 8, an inner-ring raceway 11a on the one side in the axial direction is provided around the middle section in the axial direction of the outer-circumferential direction of the main hub body 8, and a small-diameter stepped section 12 is provided on the other end section in the axial direction of the outer-circumferential surface of the main hub body 8. Here, the “one side” in the axial direction is the outside (left side in FIG. 5) in the width direction of a vehicle when assembled in a vehicle. Conversely, the center side (right side in FIG. 5) in the width direction of a vehicle when assembled in a vehicle is called the “other side”.
A center hole 13 is provided in the center section of the main hub body 8. A small-diameter section 14 is provided on the one end section in the axial direction of the center hole 13. A rod section 16 of a bolt 15, which is a connecting member for connecting the wheel-support rolling bearing unit 1 and the universal joint outer ring 2, is inserted through the small-diameter section 14 of the center hole 13 by way of a specified guiding space. An inner-ring raceway lib on the other side in the axial direction is provided around the outer-circumferential surface of the inner ring 9, and the inner ring 9 fits around the small-diameter stepped section 12 of the main hub body 8 with an interference fit. The rolling bodies 5 are arranged between the outer-ring raceways 7a, 7b and the inner-ring raceways 11a, 11b with a plurality of rolling bodies 5 rotatably arranged in each row. In the example in FIG. 5, balls are used as the rolling bodies 5, however, instead of balls it is also possible to use conical rollers as the rolling bodies 5.
A crimped section 20 is provided on the other end section in the axial direction of the main hub body 8 by causing a protruding portion of a cylindrical section 19 of the other end section in the axial direction of the main hub body 8 that protrudes toward the other side in the axial direction from an opening on the other side in the axial direction of the inner ring 9 to plastically deform outward in the radial direction. The inner ring 9 is fastened to the main hub body 8 by constraining the other end surface in the axial direction of the inner ring 9 with the one end surface in the axial direction of the crimped section 20, and this applies a proper preload to the rolling bodies 5. Hub-side face splines 21 (uneven surface in the circumferential direction) are formed around the entire circumference on the other end surface in the axial direction of the crimped section 20. In the example in FIG. 5, the tip surfaces of the teeth of the hub-side splines 21 are flat surfaces that are at a right angle with respect to the center axis of the main hub body 8.
The universal joint outer ring 2 includes a cup-shaped mouth section 22, an end wall section 23 that is a bottom section of the mouth section 22, and a cylindrical-shaped shaft section 24 that extends in one direction in the axial direction from the center section of the end wall section 23. A center hole of the shaft section 24 is a threaded hole 25. Joint-side face splines 26 (uneven surface in the circumferential direction) are formed around the entire circumference of a portion near the outside in the radial direction of the one end surface in the axial direction of the end wall section 23. In the example in FIG. 5, the tip surfaces of the teeth of the joint-side face splines 26 are flat surfaces that are at a right angle with respect to the center axis of the universal joint outer ring 2.
With the center axes of the main hub body 8 and the universal joint outer ring 2 coinciding with each other, the hub-side face splines 21 engage with the joint-side face splines 26, which makes it possible to transmit torque between the main hub body 8 and the universal joint outer ring 2. Together with inserting the rod section 16 of the bolt 15 from the one side in the axial direction through the small-diameter section 14 of the center hole 13 of the main hub body 8, a male threaded section 17 that is provided on a tip-end section of the rod section is screwed into the threaded hole 25 and further tightened. With this kind of construction, the main hub body 8 and the universal joint outer ring 2 are connected and fastened together with the main hub body 8 being held between a head section 18 of the bolt 15 and the universal joint outer ring 2.
When assembling a wheel-drive bearing unit such as described above into a vehicle, the stationary-side flange 6 of the outer ring 3 is connected and fastened to a suspension, and a wheel (drive wheel) and rotating member such as a disk for a brake are supported by and fastened to the rotating-side flange 10 of the main hub body 8. The tip-end section of a drive shaft (not illustrated in FIG. 5) that is rotated and driven by an engine by way of a transmission fits on the inside of a universal-joint inner ring 27 that is arranged on the inside of the universal-joint outer ring 2 with a spline fit. When an automobile is moving, the rotation of the universal-joint inner ring 27 is transmitted to the universal-joint outer ring 2 and main hub unit 8 by way of plural balls 28, and rotates and drives a wheel.
When assembling a wheel-support rolling bearing unit 1 of a wheel-drive bearing unit such as described above, first, together with arranging the outer ring 3 around the circumference of the main hub body 8, rolling bodies 5 are provided between the outer-ring raceway 7a on the one side in the axial direction of the outer-ring raceways 7a, 7b and the inner-ring raceway 11a on the one side in the axial direction, while being held by a retainer 29a on the one side in the axial direction. Next, rolling bodies 5 are placed around the circumference of the inner-ring raceway 11b on the other side in the axial direction that was formed around the outer-circumferential surface of the inner ring 9 while being held by a retainer 29b on the other side in the axial direction, and in this state, the inner ring 9 is fitted with an interference fit around the small-diameter stepped section 12 that was formed on the other end section in the axial direction of the main hub body 8. Together with this outer fitting work, the rolling surfaces of the rolling bodies 5 in the row on the other side in the axial direction that are held by the retainer 29b on the other side in the axial direction are brought into contact with the outer-ring raceway 7b on the other side in the axial direction that was formed around the inner-circumferential surface of a portion near the other end in the axial direction of the outer ring 3. After that, the portion of the cylindrical section 19 of the other end section in the axial direction of the main hub body 8 that protrudes toward the other side in the axial direction from the opening on the other side in the axial direction of the inner ring 9 is plastically deformed outward in the radial direction to form a crimped section 20, and by constraining in the axial direction the other end surface in the axial direction of the inner ring 9 by the one end surface in the axial direction of the crimped section 20, the inner ring 9 is fastened to the main hub body 8.
As illustrated in FIG. 6, by performing orbital forging on the other end surface in the axial direction of the crimped section 20 of the assembled wheel-support rolling bearing unit 1 using a roll 30 having a center axis that is inclined with respect to the center axis of the main hub unit 8, the hub-side face splines 21 are formed on the other end surface in the axial direction of the crimped section 20. Plural teeth that are uniformly spaced in the circumferential direction are provided on the processing surface 31, which is the tip-end surface (bottom-end surface) of the roll 30. With the processing surface 31 of the roll 30 pressed against the other end surface in the axial direction of the crimped section 20, the roll 30 is made to rotate (revolve) around the center axis of the main hub body 8, and due to the engagement between the processing surface 31 of the roll 30 and the other end surface in the axial direction of the crimped section 20, the roll 30 rotates around the center axis of the roll 30 (rotates around its own axis). From this kind of orbital forging, the hub-side face splines 21 (uneven surface in the circumferential direction) are formed on the other end surface in the axial direction of the crimped section 20.
When the hub-side face splines 21 are formed on the other end surface in the axial direction of the crimped section 20 by orbital forging, as illustrated in FIG. 6, there is a possibility that one or more radial streaks (minute concave grooves) 32 will be formed in the tip-end surfaces of the teeth of the hub-side face splines 21. In other words, when performing orbital forging, a difference may occur between the pitch in the circumferential direction of the teeth of the hub-side face splines 21 that are to be formed on the other end surface in the axial direction of the crimped section 20 and the pitch in the circumferential direction of the teeth that are formed on the processing surface 31 of the roll 30. When this difference is large, each time the roll 30 rotates around the center axis of the main hub body 8, portions of the other end surface in the axial direction of the crimped section 20 that are pressed by the teeth that are formed on the processing surface 31 of the roll 30 are moved and shifted in the circumferential direction. Then, as the same portions of the tip-end surfaces of the teeth of the hub-side face splines 21 are repeatedly pressed by the teeth of the processing surface 31 of the roll 30, the depths of the streaks 32 that are formed in the tip-end surfaces of the teeth becomes deeper. As the depths of the streaks 32 become deeper, it becomes easy for parts of the other end surface in the axial direction of the crimped section 20 to become damaged or to peel away. When the other end surface in the axial direction of the crimped section 20 becomes damaged, there is a possibility that the engaged state between the hub-side face splines 21 and the joint-side face splines 26 will become bad, and that metal powder that is generated will get inside the inner space of the wheel-support rolling bearing unit 1, causing the life of the wheel-support rolling bearing unit 1 to decrease.
WO2009/1399137 (A1) discloses a method for forming hub-side face splines on the other end surface in the axial direction of this crimped section at the same time that the crimped section is formed. However, in WO2009/139137 (A1) no study has been performed for measures to prevent the formation of radial streaks in the tip-end surfaces of the teeth of the hub-side face splines when forming the hub-side face splines.